Page:NIOSH Manual of Analytical Methods - 2520.pdf/3

 METHYL BROMIDE: METHOD 2520, Issue 2, dated 15 May 1996 - Page 3 of 5

time of transfer to autosampler vials. Dilutions can be stored in the freezer until determined that they are needed. CALIBRATION AND QUALITY CONTROL: 8.	Calibrate daily with at least six working standards over the range 0.14 to 272 µg methyl bromide per sample from calibration stock solution. NOTE:	0.085 µmoles bromine per µL is equivalent to a methyl bromide concentration of 8.07 µg/µL. a.	 Add known aliquots (2.0-, 20-, 45-, and 120-µL) of calibration stock solution to methylene chloride in 10-mL vials with PTFE-lined caps. Take 1 mL of lowest standard and dilute to 10 mL with methylene chloride. Transfer standards to autosampler vials and immediately cap each vial. b.	 Analyze together with samples and blanks (steps 11 and 12). c.	 Prepare calibration graph (peak area vs. concentration). 9.	Determine desorption efficiency (DE) at least once for each lot of charcoal used for sampling in the calibration range (step 8). Prepare three tubes at each of five levels plus three media blanks. a.	 Inject a known amount (1 to 10 µL) of methyl bromide desorption stock solution (concentration determined against other brominated compounds), or a serial dilution thereof, directly onto media blank samplers with a microliter syringe. b.	 Cap the tubes. Allow to stand overnight at 0 °C. c.	 Desorb (steps 5 through 7) and analyze together with working standards (steps 11 and 12). d.	 Prepare a graph of DE vs. µg bromine recovered. 10.	Analyze three quality control blind spikes and three analyst spikes to ensure that the calibration graph and DE graph are in control. MEASUREMENT: 11.	Set gas chromatograph-atomic emission detector to manufacturer’s recommendations and to conditions given on page 2520-1. Inject sample aliquot manually using solvent flush technique or with autosampler. NOTE 1:	Methyl bromide elutes before the methylene chloride solvent peak. Vent the solvent peak to avoid extinguishing the helium plasma. Vent time ranges from 3.2 to 5.9 min; this may need to be adjusted for each system. NOTE 2:	If peak area is above the linear range of the working standards, dilute with eluant, reanalyze, and apply the appropriate dilution factor in calculations. 12.	Measure peak area. CALCULATIONS: 13.	Determine the mass, µg (corrected for DE) of methyl bromide found in the 400-mg sample tube and 200-mg sample tube, and in the average media blanks for the 400-mg and 200-mg sorbent tubes. 14.	Calculate concentration,, of methyl bromide in the air volume sampled, (L): , mg/m³. EVALUATION OF METHOD: Method 2520 for methyl bromide, issued in August 1990 [2], was based on method S372 [5]. This issue (Issue 2, dated 5/15/96) of Method 2520 was further revised to account for the effect of humidity and to address the instability of standards and samples [1]. The addition of a drying tube to the sampling train helped reduce the effects of >50% RH. The analytical technique was changed to GC with atomic emission detection (GC-AED). The atomic emission detector monitors individual elements, in this case NIOSH Manual of Analytical Methods (NMAM), Fourth Edition